Problem: Emily is packing her bags for her vacation. She has $9$ unique action figures, but only $4$ fit in her bag. How many different groups of $4$ action figures can she take?
Emily has $4$ spaces for her action figures, so let's fill them one by one. At first, Emily has $9$ choices for what to put in the first space. For the second space, she only has $8$ action figures left, so there are only $8$ choices of what to put in the second space. So far, it seems like there are $9 \cdot 8 = 72$ different unique choices Emily could have made to fill the first two spaces in her bag. But that's not quite right. Why? Because if she picked action figure number 3, then action figure number 1, that's the same situation as picking number 1 and then number 3. They both end up in the same bag. So, if Emily keeps filling the spaces in her bag, making $9\cdot8\cdot7\cdot6 = \dfrac{9!}{(9-4)!} = 3024$ decisions altogether, we've overcounted a bunch of groups. How much have we overcounted? Well, for every group of $4$ , we've counted them as if the order we chose them in matters, when really it doesn't. So, the number of times we've overcounted each group is the number of ways to order $4$ things. There are $4! = 24$ ways of ordering $4$ things, so we've counted each group of $4$ action figures $24$ times. So, we have to divide the number of ways we could have filled the bag in order by number of times we've overcounted our groups. $ \dfrac{9!}{(9 - 4)!} \cdot \dfrac{1}{4!}$ is the number of groups of action figures Emily can bring. Another way to write this is $ \binom{9}{4} $, or $9$ choose $4$, which is $126$.